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Light-induced liquid crystallinity

Abstract

Liquid crystals are traditionally classified as thermotropic, lyotropic or polymeric, based on the stimulus that governs the organization and order of the molecular system1. The most widely known and applied class of liquid crystals are a subset of thermotropic liquid crystals known as calamitic, in which adding heat can result in phase transitions from or into the nematic, cholesteric and smectic mesophases. Photoresponsive liquid-crystal materials and mixtures can undergo isothermal phase transitions if light affects the order parameter of the system within a mesophase sufficiently. In nearly all previous examinations, light exposure of photoresponsive liquid-crystal materials and mixtures resulted in order-decreasing photo-induced isothermal phase transitions2. Under specialized conditions, an increase in order with light exposure has been reported, despite the tendency of the photoresponsive liquid-crystal system to reduce order in the exposed state3,4,5,6,7. A direct, photo-induced transition from the isotropic to the nematic phase has been observed in a mixture of spiropyran molecules and a nematic liquid crystal8. Here we report a class of naphthopyran-based materials that exhibit photo-induced conformational changes in molecular structure capable of yielding order-increasing phase transitions. Appropriate functionalization of the naphthopyran molecules leads to an exceedingly large order parameter in the open form, which results in a clear to strongly absorbing dichroic state. The increase in order with light exposure has profound implications in optics, photonics, lasing and displays and will merit further consideration for applications in solar energy harvesting. The large, photo-induced dichroism exhibited by the material system has been long sought in ophthalmic applications such as photochromic and polarized variable transmission sunglasses.

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Figure 1: Naphthopyran material properties.
Figure 2: Increasing order.
Figure 3: Clear to coloured.
Figure 4: Polarizing the light.

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Acknowledgements

T.J.W. and T.J.B. were supported by the US Air Force Office of Scientific Research and the Materials and Manufacturing Directorate of the US Air Force Office of Scientific Research. T.K., L. Sukhomlinova, L. Su and B.T acknowledge partial support through US Air Force contract FA8650-05-D-5807.

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Contributions

T.K., T.J.W., L. Sukhomlinova, L. Su, B.T. and T.J.B. designed experiments. L. Sukhomlinova conceived of and synthesized the molecules. T.K. and T.J.W. completed the experiments. T.J.W. prepared the figures and wrote the manuscript with assistance from T.K., B.T. and T.J.B.

Corresponding authors

Correspondence to Tamas Kosa or Timothy J. White.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text, Supplementary Figures 1-3 and Supplementary Table 1. (PDF 299 kb)

Supplementary Movie

This file contains Supplementary Movie 1 which shows light-induced isotropic to nematic phase transistions observed in both twisted and planarly aligned cells. (MOV 10025 kb)

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Kosa, T., Sukhomlinova, L., Su, L. et al. Light-induced liquid crystallinity. Nature 485, 347–349 (2012). https://doi.org/10.1038/nature11122

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